Page 34 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

10 −12 ) [12]. The authors also estimated that their system In general, this increase in spectral ef iciency makes
could extend to span even farther ranges with the use of higher order modulations the most attractive, due to the
more advanced error‑correction codes. fact that more information can be sent within in a given
bandwidth or, conversely, that the system requires less
Collectively, these demonstrations indicate that long‑ bandwidth to maintain a given data rate.
range terahertz communication links are not only possi‑
ble, but will likely be implemented commercially in the
foreseeable future as the technology progresses. This However, higher‑order modulations are not always viable
technology would provide many bene its, since there are to use. When a system is constrained to operate below
many situations in which the ability to rapidly estab‑ some ixed maximum power, the phase and amplitude
of all communication symbols must fall within a inite
lish a directive, wireless point‑to‑point link with a ca‑
region of the phase/amplitude plane that satis ies that
pacity of tens of Gb/s would be highly attractive, inclu-
power constraint. Increasing the number of communica‑
ding temporary installments during disaster recovery
or wartime environments, at locations where trenching tion symbols necessarily means that symbols must take
iber is prohibitively expensive or time‑consuming, or as on increasingly similar values of amplitude and/or phase,
a re‑ placement to upgrade microwave point‑to‑point as more symbols have to be placed within the inite re‑
backhaul links. gion satisfying the power constraint. When the receiver
must differentiate between a large number of similar
There are many design choices that must be considered
symbols with high resolution, injected noise can easily
when planning the construction of such a link [13, 14, 15,
shift the amplitude and/or phase of the received
16]. One notable known design choice is selecting the
waveform so that symbols are received in error, much
modulation scheme, since modulation type determines
more so than for a lower‑order modulation scheme where
the shape of the temporal waveform, the hardware re‑
symbol regions are larger and more widely spaced. As
quirements (for example, the dynamic range of the front‑
a result, higher‑order M‑QAM schemes have more
end receiver), the resilience of the channel to interference,
stringent requirements on the minimum SNR allowed at
and the achievable throughput of the channel. By judi‑
the receiver for effective operation. Fig. 1 illustrates the
cious selection of the modulation type, channel through‑
increase in SNR required by higher‑order M‑QAM
put can be maximized, and many research teams have
schemes in order to maintain a given bit error rate.
investigated various algorithms and strategies for
determining the optimal modulations for both
microwave and terahertz wireless links [17, 18, 19]. When designing a communication link, the modulation
type is chosen so that an acceptable error rate is main‑
Many modulation schemes are possible, and all carry their
tained under the worst‑case SNR the link is designed to
own its and drawbacks. However, the prototype
handle. In order to decrease the outage probability du-
terahertz links in the demonstrations listed earlier em‑ ploy
ring times when the signal is strongly attenuated, and
various orders of quadrature amplitude modulation,
to increase the capacity when channel conditions are
collectively known as M‑QAM schemes, including Binary
favorable, many communication systems employ
Phase Shift Keying (BPSK, or 2‑QAM), Quadrature Phase
optimization algorithms that actively select the order of
Shift Keying (QPSK, or 4‑QAM), 8‑QAM and 16‑QAM. In an
the modulation scheme used [17]. These optimization
M‑QAM scheme, binary data is encoded as communi‑ cation
routines switch between modulation orders as channel
symbols, distinct combinations of amplitude and phase of
conditions vary, such that the resulting link is both more
the carrier wave, each of which represent one or more bits
reliable (in terms of outage probability) and operates
of data. The modulation order M speci ies how many such
with a higher average capacity.
combinations of amplitude and phase are recognized by the
receiver, and log ( ) bits of data are carried by each
2
symbol.
Terahertz links will, of course, likewise bene it from these
As the modulation order of the communication system is type of optimization routines [18, 17], whether the band‑
increased, each symbol transition carries more infor‑ width is occupied by a single link, or illed with a large
mation, which consequently increases the spectral ef i‑ number of subcarriers [21]. However, due to the huge
ciency of the link. Spectral ef iciency is a measure of how bandwidths available for terahertz communication links,
many bits of data are transferred per unit of bandwidth and the high frequencies at which they operate, the opti‑
utilized by the communication system, typically given in mal modulation type will not be determined by SNR (that
units of ( bits ) is, fading) alone. Our work indicates that the Group Velo-
/ Hz. While the spectral ef iciency realized in a
physical communication system depends on many factors city Dispersion (GVD) caused by molecular resonances
(such as the coding scheme, Signal‑To‑Noise Ratio (SNR), in the atmosphere can result in counter‑intuitive
and fading characteristics of the channel), the theoretical behavior over the lower terahertz bands, in which the
maximum spectral ef iciency of an M‑QAM scheme is ulti‑ severity of Inter‑Symbol Interference (ISI) depends not
mately given by, and scales with modulation order only on band‑ width (as expected), but also on the
modulation type used, even in the absence of noise.
according to, log2( ) [20].
22 © International Telecommunication Union, 2021